Oxide-based semiconductor non-linear element having gate electrode electrically connected to source or drain electrode

ABSTRACT

A non-linear element (e.g., a diode) with small reverse saturation current is provided. A non-linear element includes a first electrode provided over a substrate, an oxide semiconductor film provided on and in contact with the first electrode, a second electrode provided on and in contact with the oxide semiconductor film, a gate insulating film covering the first electrode, the oxide semiconductor film, and the second electrode, and a third electrode provided in contact with the gate insulating film and adjacent to a side surface of the oxide semiconductor film with the gate insulating film interposed therebetween or a third electrode provided in contact with the gate insulating film and surrounding the second electrode. The third electrode is connected to the first electrode or the second electrode.

TECHNICAL FIELD

The present invention relates to a non-linear element including an oxidesemiconductor and a semiconductor device including the non-linearelement, such as a display device. Furthermore, the present inventionrelates to an electronic device including the semiconductor device.

BACKGROUND ART

Among semiconductor devices, diodes are required to have high withstandvoltage, small reverse saturation current, and the like. In order tomeet such a requirement, a diode in which silicon carbide (SiC) is usedhas been researched. Silicon carbide used as a semiconductor materialhas a width of a forbidden band of 3 eV or more, excellentcontrollability of electric conductivity at high temperature, and ismore resistant to dielectric breakdown than silicon. Therefore, siliconcarbide is expected to be applied to a diode in which reverse saturationcurrent is small and withstand voltage is high. For example, a Schottkybarrier diode in which silicon carbide is used and reverse leakagecurrent is reduced is known (Patent Document 1).

However, in the case of using silicon carbide, it is difficult to obtaincrystals with good quality, and further, a device can be fabricated onlyat high process temperature. For example, an ion implantation method isused to form an impurity region in silicon carbide; in that case, heattreatment at 1500° C. or higher is necessary in order to activate adopant or repair crystal defects caused by ion implantation.

In addition, since carbon is contained as a component in siliconcarbide, an insulating film with good quality cannot be formed bythermal oxidation. Furthermore, silicon carbide is chemically verystable and is not easily etched by normal wet etching.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2000-133819

DISCLOSURE OF INVENTION

As described above, although a non-linear element (e.g., a diode) inwhich silicon carbide is used is expected to have high withstand voltageand small reverse saturation current, there are many problems inmanufacturing and achieving such an element.

In view of the above, it is an object of an embodiment of the presentinvention to provide a non-linear element with small reverse saturationcurrent. In addition, it is an object to manufacture a non-linearelement with small reverse saturation current at low process temperature(e.g., less than or equal to 800° C.).

An embodiment of the present invention provides a non-linear element(e.g., a diode) which can be miniaturized and includes a field effecttransistor (for example, a thin film transistor) that can bemanufactured at low process temperature and has large on-state currentand small off-state current. A non-linear element includes a firstelectrode provided over a substrate, an oxide semiconductor filmprovided on and in contact with the first electrode and purified, asecond electrode provided on and in contact with the oxide semiconductorfilm, a gate insulating film covering the first electrode, the oxidesemiconductor film, and the second electrode, and third electrodesprovided in contact with the gate insulating film and facing each otherwith the first electrode, the oxide semiconductor film, and the secondelectrode interposed therebetween or a third electrode provided incontact with the gate insulating film and surrounding the secondelectrode. In the non-linear element, the third electrodes or the thirdelectrode are/is connected to the first electrode or the secondelectrode, and a current flows between the first electrode and thesecond electrode.

With a field effect transistor (for example, a thin film transistor)which can be miniaturized and has large on-state current and smalloff-state current, it is possible to obtain a diode which has very smallreverse current. Accordingly, a diode which is resistant to a breakdown(i.e., has high withstand voltage) can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are a top view and a cross-sectional view illustrating adiode which is one embodiment of the present invention.

FIGS. 2A and 2B are a top view and a cross-sectional view illustrating adiode which is one embodiment of the present invention.

FIGS. 3A and 3B are a top view and a cross-sectional view illustrating adiode which is one embodiment of the present invention.

FIGS. 4A and 4B are a top view and a cross-sectional view illustrating adiode which is one embodiment of the present invention.

FIGS. 5A and 5B are a top view and a cross-sectional view illustrating adiode which is one embodiment of the present invention.

FIGS. 6A and 6B are a top view and a cross-sectional view illustrating adiode which is one embodiment of the present invention.

FIGS. 7A to 7E are cross-sectional views illustrating a method formanufacturing a diode which is one embodiment of the present invention.

FIGS. 8A and 8B are cross-sectional views illustrating a method formanufacturing a diode which is one embodiment of the present invention.

FIG. 9 is a diagram illustrating a display device which is oneembodiment of the present invention.

FIGS. 10A to 10F are diagrams each illustrating a protection circuitprovided in a display device which is one embodiment of the presentinvention.

FIGS. 11A to 11C are diagrams each illustrating an electronic devicewhich is one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail withreference to the drawings. Note that the present invention is notlimited to the description below, and it will be easily understood bythose skilled in the art that modes and details thereof can be changedin various ways without departing from the spirit and the scope of thepresent invention. Therefore, the present invention should not beconstrued as being limited to the description of the embodiments. Notethat in structures of the present invention described hereinafter, likeportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description thereof is notrepeated.

Note that in each drawing described in this specification, the size ofeach component or the thickness of each layer or an area is exaggeratedin some cases for clarification. Therefore, embodiments of the presentinvention are not limited to such scales.

Note that terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, description can be made even when “first” is replaced with“second” or “third”, as appropriate.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential) is simplycalled a potential or a voltage, and a potential and a voltage are usedas synonymous words in many cases. Thus, in this specification, apotential may be rephrased as a voltage and a voltage may be rephrasedas a potential unless otherwise specified.

Embodiment 1

In this embodiment, an example of a structure of a diode which is oneembodiment of the present invention will be described with reference toFIGS. 1A and 1B. The diode which is described in this embodiment can beobtained by connecting a source or a drain of a field effect transistor,for example, a thin film transistor to a gate thereof.

In the diode illustrated in FIGS. 1A and 1B, a wiring 125 is connectedto a third electrode 113, a third electrode 115, and a second electrode109, and the second electrode 109 is connected to a first electrode 105through an oxide semiconductor film 107. The first electrode 105 isconnected to a wiring 131.

FIG. 1A is a top view of a diode-connected thin film transistor 133.FIG. 1B is a cross-sectional view along dashed-and-dotted line A-B inFIG. 1A.

As illustrated in FIG. 1B, the first electrode 105, the oxidesemiconductor film 107, and the second electrode 109 are stacked over aninsulating film 103 which is formed over a substrate 101. A gateinsulating film 111 is provided so as to cover the first electrode 105,the oxide semiconductor film 107, and the second electrode 109. Thethird electrode 113 and the third electrode 115 are provided over thegate insulating film 111. An insulating film 117 functioning as aninterlayer insulating film is provided over the gate insulating film111, the third electrode 113, and the third electrode 115. Openings areformed in the gate insulating film 111 and the insulating film 117, andthe wiring 131 (see FIG. 1A) connected to the first electrode 105 andthe wiring 125 connected to the second electrode 109, the thirdelectrode 113, and the third electrode 115 are formed in the openings.The first electrode 105 functions as one of a source electrode and adrain electrode of the thin film transistor. The second electrode 109functions as the other of the source electrode and the drain electrodeof the thin film transistor. The third electrode 113 and the thirdelectrode 115 function as a gate electrode of the thin film transistor.

The thin film transistor according to this embodiment is a vertical thinfilm transistor, which has features that the third electrode 113 and thethird electrode 115 which function as a gate electrode are separated andthat the third electrode 113 and the third electrode 115 face each otherwith the first electrode 105, the oxide semiconductor film 107, and thesecond electrode 109 interposed therebetween.

Note that a thin film transistor is an element that includes at leastthree terminals, including a gate, a drain, and a source. The thin filmtransistor includes a channel formation region between a drain regionand a source region, and current can flow through the drain region, thechannel formation region, and the source region. Here, since the sourceand the drain of the thin film transistor may change depending on astructure, operating conditions, and the like of the thin filmtransistor, it is difficult to define which is a source or a drain.Therefore, a region functioning as a source and a drain is not calledthe source or the drain in some cases. In such a case, for example, oneof the source and the drain may be referred to as a first terminal andthe other may be referred to as a second terminal. Alternatively, one ofthe source and the drain may be referred to as a first electrode and theother may be referred to as a second electrode. Further alternatively,one of the source and the drain may be referred to as a first region andthe other may be referred to a second region.

It is necessary that the substrate 101 at least have heat resistancesufficient to withstand heat treatment to be performed later. As thesubstrate 101, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like can be used.

As the glass substrate, in the case where the temperature of the heattreatment to be performed later is high, the one whose strain point is730° C. or higher is preferably used. As a glass substrate, a glassmaterial such as aluminosilicate glass, aluminoborosilicate glass, orbarium borosilicate glass is used, for example. Note that in general, bycontaining a larger amount of barium oxide (BaO) than boron oxide, aglass substrate which is heat-resistant and more practical can beobtained. Therefore, a glass substrate containing BaO and B₂O₃ so thatthe amount of BaO is larger than that of B₂O₃ is preferably used.

Note that a substrate formed of an insulator, such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate, may be used,instead of the glass substrate. Alternatively, a crystallized glasssubstrate or the like may be used.

The insulating film 103 is formed using an oxide insulating film such asa silicon oxide film or a silicon oxynitride film, or a nitrideinsulating film such as a silicon nitride film, a silicon nitride oxidefilm, an aluminum nitride film, or an aluminum nitride oxide film. Inaddition, the insulating film 103 may have a stacked structure, forexample, a stacked structure in which one or more of the nitrideinsulating films and one or more of the oxide insulating film arestacked in that order over the substrate 101.

The first electrode 105 and the second electrode 109 are formed using anelement selected from aluminum, chromium, copper, tantalum, titanium,molybdenum, tungsten, and yttrium, an alloy containing any of theseelements as a component, an alloy containing any of these elements incombination, or the like. Alternatively, one or more materials selectedfrom manganese, magnesium, zirconium, beryllium, and thorium can beused. In addition, the first electrode 105 can have a single-layerstructure or a stacked structure having two or more layers. For example,a single-layer structure of an aluminum film containing silicon, atwo-layer structure of an aluminum film and a titanium film stackedthereover, a two-layer structure of a tungsten film and a titanium filmstacked thereover, a three-layer structure in which a titanium film, analuminum film, and a titanium film are stacked in that order, and thelike can be given. Alternatively, a film, an alloy film, or a nitridefilm which contains aluminum and one or more elements selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

As the oxide semiconductor film 107, a thin film of a material expressedby InMO₃(ZnO)_(m) (m>0, where m is not an integer) can be used. Here, Mrepresents one or more metal elements selected from Ga, Fe, Ni, Mn, andCo. For example, M may be Ga, Ga and Ni, Ga and Fe, or the like. Theoxide semiconductor film may contain a transition metal element or anoxide of the transition metal element as an impurity element in additionto the metal element contained as M. An oxide semiconductor whosecomposition formula is represented as InMO₃ (ZnO)_(m) (m>0, where m isnot an integer) where Ga is contained as M is referred to as anIn—Ga—Zn—O-based oxide semiconductor, and a thin film thereof isreferred to as an In—Ga—Zn—O-based film.

As the oxide semiconductor film 107, any of the following oxidesemiconductor films can be used besides the In—Ga—Zn—O-based oxidesemiconductor film: an In—Sn—Zn—O-based oxide semiconductor film; anIn—Al—Zn—O-based oxide semiconductor film; a Sn—Ga—Zn—O-based oxidesemiconductor film; an Al—Ga—Zn—O-based oxide semiconductor film; aSn—Al—Zn—O-based oxide semiconductor film; an In—Zn—O-based oxidesemiconductor film; a Sn—Zn—O-based oxide semiconductor film; anAl—Zn—O-based oxide semiconductor film; an In—O-based oxidesemiconductor film; a Sn—O-based oxide semiconductor film; and aZn—O-based oxide semiconductor film. Further, Si may be contained in theabove oxide semiconductor film.

In the oxide semiconductor film 107 used in this embodiment, hydrogen iscontained at 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸ atoms/cm³ orless, more preferably 5×10¹⁷ atoms/cm³ or less, and hydrogen is removedfrom the oxide semiconductor film. In other words, the oxidesemiconductor film is purified so that impurities that are not maincomponents of the oxide semiconductor film are contained as little aspossible. The carrier concentration of the oxide semiconductor film 107is 5×10¹⁴ atoms/cm³ or less, preferably 1×10¹⁴ atoms/cm³ or less, morepreferably 5×10¹² atoms/cm³ or less, still more preferably 1×10¹²atoms/cm³ or less. That is, the carrier concentration of the oxidesemiconductor film is close to zero. Furthermore, the energy gap is 2 eVor more, preferably 2.5 eV or more, more preferably 3 eV or more. Notethat the hydrogen concentration of the oxide semiconductor film can bemeasured by secondary ion mass spectrometry (SIMS). In addition, thecarrier density can be measured by the Hall effect measurement.

The thickness of the oxide semiconductor film 107 may be 30 nm to 3000nm. When the thickness of the oxide semiconductor film 107 is small, thechannel length of the thin film transistor can be decreased; thus, athin film transistor having large on current and high field-effectmobility can be manufactured. On the other hand, when the thickness ofthe oxide semiconductor film 107 is large, typically 100 nm to 3000 nm,a high-power semiconductor device can be manufactured.

The gate insulating film 111 can be a single-layer or a stack formedusing any of a silicon oxide film, a silicon nitride film, a siliconoxynitride film, a silicon nitride oxide film, and an aluminum oxidefilm. A portion of the gate insulating film 111 which is in contact withthe oxide semiconductor film 107 preferably contains oxygen, and inparticular, the portion of the gate insulating film 111 is preferablyformed using a silicon oxide film. By using a silicon oxide film, oxygencan be supplied to the oxide semiconductor film 107 and favorablecharacteristics can be obtained. The thickness of the gate insulatingfilm 111 may be 50 nm to 500 nm. When the thickness of the gateinsulating film 111 is small, a thin film transistor having highfield-effect mobility can be manufactured; thus, a driver circuit can bemanufactured over the same substrate as the thin film transistor. On theother hand, when the thickness of the gate insulating film 111 is large,gate leakage current can be reduced.

When the gate insulating film 111 is formed using a high-k material suchas hafnium silicate (HfSiO_(x) (x>0)), HfSiO_(x) (x>0) to which N isadded, hafnium aluminate (HfAlO_(x) (x>0)), hafnium oxide, or yttriumoxide, gate leakage can be reduced. Further, a stacked structure can beused in which a high-k material and one or more of a silicon oxide film,a silicon nitride film, a silicon oxynitride film, a silicon nitrideoxide film, and an aluminum oxide film are stacked.

The third electrode 113 and the third electrode 115 which function as agate electrode are formed using an element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, and tungsten, an alloycontaining any of these elements as a component, an alloy containing anyof these elements in combination, or the like. Alternatively, one ormore materials selected from manganese, magnesium, zirconium, andberyllium may be used. In addition, the third electrode 113 and thethird electrode 115 can have a single-layer structure or a stackedstructure having two or more layers. For example, a single-layerstructure of an aluminum film containing silicon, a two-layer structureof an aluminum film and a titanium film stacked thereover, a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in that order, and the like can be given.Alternatively, a film, an alloy film, or a nitride film which containsaluminum and one or more elements selected from titanium, tantalum,tungsten, molybdenum, chromium, neodymium, and scandium may be used.

The oxide semiconductor film in this embodiment is an intrinsic (i-type)or substantially intrinsic oxide semiconductor film obtained by removalof hydrogen, which is an n-type impurity, from the oxide semiconductorfilm and the increase in purity so that an impurity other than the maincomponents of the oxide semiconductor film is not included as much aspossible. In other words, the oxide semiconductor film in thisembodiment is a purified intrinsic (i-type) oxide semiconductor film oran oxide semiconductor film which is close to a purified intrinsic oxidesemiconductor film obtained not by addition of an impurity but byremoval of an impurity such as hydrogen, water, a hydroxyl group, orhydride as much as possible. In this manner, the Fermi level (E_(f)) canbe at the same level as the intrinsic Fermi level (E_(i)).

By removing the impurity as much as possible as described above, forexample, even when the channel width W of the thin film transistor is1×10⁴ μm and the channel length thereof is 3 μm, off current can be lessthan or equal to 10⁻¹³ A, which is extremely small, and a subthresholdswing (S value) can be 0.1 V/dec (the gate insulating film with athickness of 100 nm).

As described above, when the oxide semiconductor film is purified sothat impurities that are not main components of the oxide semiconductorfilm, typically hydrogen, water, a hydroxyl group, or hydride, arecontained as little as possible, favorable operation of the thin filmtransistor can be obtained. In particular, off current can be reduced.

In a lateral thin film transistor in which a channel is formedsubstantially in parallel with a substrate, a source and a drain as wellas the channel need to be provided laterally, so that an area occupiedby the thin film transistor in the substrate is increased, which hindersminiaturization. However, in a vertical thin film transistor, a source,a channel, and a drain are stacked, whereby an area occupied by the thinfilm transistor in a substrate surface can be reduced. As a result ofthis, it is possible to miniaturize the thin film transistor.

The channel length of the vertical thin film transistor can becontrolled by the thickness of the oxide semiconductor film; therefore,when the oxide semiconductor film 107 is formed to have a smallthickness, a thin film transistor having a short channel length can beprovided. When the channel length is reduced, series resistance of thesource, the channel, and the drain can be reduced; therefore, on currentand field-effect mobility of the thin film transistor can be increased.In addition, a thin film transistor having the purified oxidesemiconductor film whose hydrogen concentration is reduced is in aninsulating state where off current is extremely small and almost nocurrent flows when the thin film transistor is off. Therefore, even whenthe thickness of the oxide semiconductor film is decreased to reduce thechannel length of the vertical thin film transistor, a thin filmtransistor in which almost no off current flows in a non-conductionstate can be provided.

As described above, using a purified oxide semiconductor film whosehydrogen concentration is reduced makes it possible to manufacture athin film transistor which is suitable for higher definition, has highoperation speed, and is capable of conducting a large amount of currentin an on state and almost no current in an off state.

Note that the diode described in this embodiment is not limited to thatillustrated in FIGS. 1A and 1B. In the diode illustrated in FIGS. 1A and1B, current flows through the oxide semiconductor film 107 from thesecond electrode 109 to the first electrode 105. A structure in whichcurrent flows through the oxide semiconductor film 107 from the firstelectrode 105 to the second electrode 109 as illustrated in FIGS. 2A and2B may be employed.

In a diode illustrated in FIGS. 2A and 2B, a wiring 125 is connected toa third electrode 113, a third electrode 115, and a first electrode 105.The first electrode 105 is connected to a second electrode 109 throughan oxide semiconductor film 107. The second electrode 109 is connectedto a wiring 131.

In the diode illustrated in FIGS. 2A and 2B, a wiring 125 is provided soas not to overlap with other electrodes and the like; therefore,parasitic capacitance generated between the wiring 125 and otherelectrodes can be suppressed.

By connecting a source or a drain of a thin film transistor to a gatethereof as described above, a diode in which reverse current is verysmall can be obtained. Therefore, a diode which is resistant to abreakdown (i.e., has high withstand voltage) can be manufactured.

Embodiment 2

In this embodiment, an example of a diode having a structure differentfrom that in Embodiment 1 will be described with reference to FIGS. 3Aand 3B. The diode which is described in this embodiment can be obtainedby connecting a source or a drain of a field effect transistor, forexample, a thin film transistor to a gate thereof.

In the diode illustrated in FIGS. 3A and 3B, a wiring 131 is connectedto a first electrode 105 and a third electrode 113, and a wiring 132 isconnected to a first electrode 106 and a third electrode 115. The firstelectrode 105 and the first electrode 106 are connected to a secondelectrode 109 through an oxide semiconductor film 107. The secondelectrode 109 is connected to a wiring 129.

FIG. 3A is a top view of diode-connected thin film transistors 141 and143. FIG. 3B is a cross-sectional view along dashed-and-dotted line A-Bin FIG. 3A.

As illustrated in FIG. 3B, the first electrode 105 and the firstelectrode 106, the oxide semiconductor film 107, and the secondelectrode 109 are stacked over an insulating film 103 which is formedover a substrate 101. A gate insulating film 111 is provided so as tocover the first electrode 105, the first electrode 106, the oxidesemiconductor film 107, and the second electrode 109. The thirdelectrode 113 and the third electrode 115 are provided over the gateinsulating film 111. An insulating film 117 functioning as an interlayerinsulating film is provided over the gate insulating film 111, the thirdelectrode 113, and the third electrode 115. Openings are formed in theinsulating film 117. The wiring 131 connected to the first electrode 105and the third electrode 113 each through the opening, the wiring 132connected to the first electrode 106 and the third electrode 115 eachthrough the opening (see FIG. 3A), and the wiring 129 connected to thesecond electrode 109 through the opening are formed.

The first electrode 105 functions as one of a source electrode and adrain electrode of the thin film transistor 141. The first electrode 106functions as one of a source electrode and a drain electrode of the thinfilm transistor 143. The second electrode 109 functions as the other ofthe source electrode and the drain electrode of each of the thin filmtransistors 141 and 143. The third electrode 113 functions as a gateelectrode of the thin film transistor 141. The third electrode 115functions as a gate electrode of the thin film transistor 143.

A feature of this embodiment is that the first electrode 105 and thefirst electrode 106 are separated from each other (see FIGS. 3A and 3B).

Furthermore, a feature is that the thin film transistor 141 and the thinfilm transistor 143 are connected in parallel by the second electrode109 and the wiring 129 in FIGS. 3A and 3B. In that case, the firstelectrode 105 functions as one of the source electrode and the drainelectrode (e.g., the source) of the thin film transistor 141. The secondelectrode 109 functions as the other of the source electrode and thedrain electrode (e.g., the drain) of the thin film transistor 141. Thethird electrode 113 functions as the gate electrode of the thin filmtransistor 141. The second electrode 109 also functions as one of thesource electrode and the drain electrode (e.g., the drain) of the thinfilm transistor 143. The first electrode 106 functions as the other ofthe source electrode and the drain electrode (e.g., the source) of thethin film transistor 143. The third electrode 115 functions as the gateelectrode of the thin film transistor 143.

Alternatively, the thin film transistor 141 and the thin film transistor143 may be connected in series. In other words, the thin film transistor141 and the thin film transistor 143 are connected in series by thesecond electrode 109. In that case, the wiring 129 is not necessarilyprovided. In that case, a diode may be configured to output a signalthrough the wiring 132.

In the case where the thin film transistor 141 and the thin filmtransistor 143 are connected in series by the second electrode 109, thefirst electrode 105 functions as one of the source electrode and thedrain electrode (e.g., the source) of the thin film transistor 141. Thesecond electrode 109 functions as the other of the source electrode andthe drain electrode (e.g., the drain) of the thin film transistor 141.The third electrode 113 functions as the gate electrode of the thin filmtransistor 141. The second electrode 109 also functions as one of thesource electrode and the drain electrode (e.g., the source) of the thinfilm transistor 143. The first electrode 106 functions as the other ofthe source electrode and the drain electrode (e.g., the drain) of thethin film transistor 143. The third electrode 115 functions as the gateelectrode of the thin film transistor 143.

The thin film transistors 141 and 143 of this embodiment are formedusing a purified oxide semiconductor film whose hydrogen concentrationis reduced, in a manner similar to that of Embodiment 1. Therefore,favorable operation of the thin film transistors can be obtained. Inparticular, off current can be reduced. As a result of this, a thin filmtransistor which is suitable for higher definition, has high operationspeed, and is capable of conducting a large amount of current in an onstate and almost no current in an off state can be manufactured.

Note that the diode described in this embodiment is not limited to thatillustrated in FIGS. 3A and 3B. In the diode illustrated in FIGS. 3A and3B, current flows through the oxide semiconductor film 107 from thefirst electrode 105 and the first electrode 106 to the second electrode109. A structure in which current flows through the oxide semiconductorfilm 107 from the second electrode 109 to the first electrode 105 andthe first electrode 106 as illustrated in FIGS. 4A and 4B may beemployed.

In the diode illustrated in FIGS. 4A and 4B, a wiring 125 is connectedto a third electrode 113, a third electrode 115, and a second electrode109. The second electrode 109 is connected to a first electrode 105 anda first electrode 106 through an oxide semiconductor film 107. The firstelectrode 105 is connected to a wiring 131, and the first electrode 106is connected to a wiring 132.

In the diode illustrated in FIGS. 4A and 4B, the wiring 125 is providedso as to overlap with a thin film transistor 141 and a thin filmtransistor 143. However, without limitation thereto, the wiring 125 maybe provided so as not to overlap with the thin film transistor 141 andthe thin film transistor 143 as in FIGS. 2A and 2B. When the wiring 125does not overlap with the thin film transistor 141 and the thin filmtransistor 143, parasitic capacitance generated between the wiring 125and electrodes of the thin film transistors can be suppressed.

By connecting a source or a drain of a thin film transistor to a gatethereof as described above, a diode in which reverse current is verysmall can be obtained. Therefore, a diode which is resistant to abreakdown (i.e., has high withstand voltage) can be manufactured.

Embodiment 3

In this embodiment, an example of a diode, which is an embodiment of thepresent invention and has a structure different from those inEmbodiments 1 and 2, will be described with reference to FIGS. 5A and5B. The diode which is described in this embodiment can be obtained byconnecting a source or a drain of a field effect transistor, forexample, a thin film transistor to a gate thereof.

In the diode illustrated in FIGS. 5A and 5B, a wiring 131 is connectedto a first electrode 105 and a third electrode 113. The first electrode105 is connected to a second electrode 109 through an oxidesemiconductor film 107. The second electrode 109 is connected to awiring 129.

FIG. 5A is a top view of a diode-connected thin film transistor 145.FIG. 5B is a cross-sectional view along dashed-and-dotted line A-B inFIG. 5A.

As illustrated in FIG. 5B, the first electrode 105, the oxidesemiconductor film 107, and the second electrode 109 are stacked over aninsulating film 103 formed over a substrate 101. A gate insulating film111 is provided so as to cover the first electrode 105, the oxidesemiconductor film 107, and the second electrode 109. The thirdelectrode 113 is provided over the gate insulating film 111. Theinsulating film 117 functioning as an interlayer insulating film isprovided over the gate insulating film 111 and the third electrode 113.Openings are formed in the insulating film 117. The wiring 131 connectedto the first electrode 105 and the third electrode 113 each through theopening (see FIG. 5A), and a wiring 129 connected to the secondelectrode 109 through the opening are formed.

The first electrode 105 functions as one of a source electrode and adrain electrode of the thin film transistor 145. The second electrode109 functions as the other of the source electrode and the drainelectrode of the thin film transistor 145. The third electrode 113functions as a gate electrode of the thin film transistor 145.

In this embodiment, the third electrode 113 functioning as the gateelectrode has a ring shape. When the third electrode 113 functioning asthe gate electrode has a ring shape, the channel width of the thin filmtransistor can be increased. Accordingly, on current of the thin filmtransistor can be increased.

The thin film transistor 145 of this embodiment is formed using apurified oxide semiconductor film whose hydrogen concentration isreduced, in a manner similar to that of Embodiment 1. Therefore,favorable operation of the thin film transistor can be obtained. Inparticular, off current can be reduced. As a result of this, a thin filmtransistor which is suitable for higher definition, has high operationspeed, and is capable of conducting a large amount of current in an onstate and almost no current in an off state can be manufactured.

Note that the diode described in this embodiment is not limited to thatillustrated in FIGS. 5A and 5B. In the diode illustrated in FIGS. 5A and5B, current flows through the oxide semiconductor film 107 from thefirst electrode 105 to the second electrode 109. A structure in whichcurrent flows through the oxide semiconductor film 107 from the secondelectrode 109 to the first electrode 105 as illustrated in FIGS. 6A and6B may be employed.

In the diode illustrated in FIGS. 6A and 6B, a wiring 129 is connectedto a second electrode 109 and a third electrode 113. The secondelectrode 109 is connected to a first electrode 105 through an oxidesemiconductor film 107. The first electrode 105 is connected to a wiring131.

By connecting a source or a drain of a thin film transistor to a gatethereof as described above, a diode in which reverse current is verysmall can be obtained. Therefore, a diode which is resistant to abreakdown (i.e., has high withstand voltage) can be manufactured.

Embodiment 4

In this embodiment, a manufacturing process of the diode-connected thinfilm transistor in FIGS. 1A and 1B will be described with reference toFIGS. 7A to 7E.

As illustrated in FIG. 7A, the insulating film 103 is formed over thesubstrate 101, and the first electrode 105 is formed over the insulatingfilm 103. The first electrode 105 functions as one of the sourceelectrode and the drain electrode of the thin film transistor.

The insulating film 103 can be formed by a sputtering method, a CVDmethod, a coating method, or the like.

Note that when the insulating film 103 is formed by a sputtering method,the insulating film 103 is preferably formed while hydrogen, water, ahydroxyl group, hydride, or the like remaining in a treatment chamber isremoved. This is for preventing hydrogen, water, a hydroxyl group,hydride, or the like from being contained in the insulating film 103. Itis preferable to use an entrapment vacuum pump in order to removehydrogen, water, a hydroxyl group, hydride, or the like remaining in thetreatment chamber. As the entrapment vacuum pump, a cryopump, an ionpump, or a titanium sublimation pump is preferably used, for example.Further, as an evacuation unit, a cold trap may be added to a turbopump. Since impurities, particularly, hydrogen, water, a hydroxyl group,hydride, or the like are removed from the treatment chamber which isevacuated using a cryopump, when the insulating film 103 is formed inthe treatment chamber, the concentration of impurities contained in theinsulating film 103 can be reduced.

As a sputtering gas used for forming the insulating film 103, a highpurity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to a concentration of 1ppm or lower (preferably, 10 ppb or lower). Note that the sputtering gasmeans a gas which is introduced into a treatment chamber wheresputtering is performed.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power source, aDC sputtering method in which a direct current power source is used, anda pulsed DC sputtering method in which a bias is applied in a pulsedmanner. The RF sputtering method is mainly used in the case where aninsulating film is formed, whereas the DC sputtering method is mainlyused in the case where a metal film is formed.

There is also a multi-source sputtering apparatus in which a pluralityof targets of different materials can be set. With the multi-sourcesputtering apparatus, films of different materials can be formed to bestacked in the same chamber, or films of plural kinds of materials canbe formed by electric discharge at the same time in the same chamber.

Alternatively, a sputtering apparatus provided with a magnet systeminside the chamber and used for a magnetron sputtering method, or asputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge can be used.

Further, as a sputtering method, a reactive sputtering method in which atarget substance and a sputtering gas component are chemically reactedwith each other during deposition to form a thin compound film thereof,or a bias sputtering method in which voltage is also applied to asubstrate during deposition can be used.

As the sputtering in this specification, the above-described sputteringapparatus and the sputtering method can be employed as appropriate.

In this embodiment, the substrate 101 is transferred to the treatmentchamber. A sputtering gas containing high purity oxygen, from whichhydrogen, water, a hydroxyl group, hydride, or the like is removed, isintroduced into the treatment chamber, and a silicon oxide film isformed as the insulating film 103 over the substrate 101 using a silicontarget. Note that when the insulating film 103 is formed, the substrate101 may be heated.

For example, the silicon oxide film is formed by an RF sputtering methodunder the following conditions: quartz (preferably, synthesized quartz)is used; the substrate temperature is 108° C.; the distance between thetarget and the substrate (the T-S distance) is 60 mm; the pressure is0.4 Pa; the high frequency power source is 1.5 kW; and the atmospherecontains oxygen and argon (the flow ratio of oxygen to argon is 1:1(each flow rate is 25 sccm)). The film thickness may be 100 nm, forexample. Note that instead of quartz (preferably, synthesized quartz), asilicon target can be used. Note that as the sputtering gas, oxygen, ora mixed gas of oxygen and argon is used.

For example, when the insulating film 103 is formed using a stackedstructure, a silicon nitride film is formed using a silicon target and asputtering gas containing high purity nitrogen from which hydrogen,water, a hydroxyl group, hydride, or the like is removed, between thesilicon oxide film and the substrate. Also in this case, it ispreferable that a silicon nitride film be formed while hydrogen, water,a hydroxyl group, hydride, or the like remaining in the treatmentchamber is removed in a manner similar to the case of the silicon oxidefilm. Note that in the process, the substrate 101 may be heated.

When a silicon nitride film and a silicon oxide film are stacked as theinsulating film 103, the silicon nitride film and the silicon oxide filmcan be formed using a common silicon target in the same treatmentchamber. First, a sputtering gas containing nitrogen is introduced intothe treatment chamber, and a silicon nitride film is formed using asilicon target provided in the treatment chamber; next, the sputteringgas containing nitrogen is switched to a sputtering gas containingoxygen, and a silicon oxide film is formed using the same silicontarget. The silicon nitride film and the silicon oxide film can beformed in succession without being exposed to air; therefore, impuritiessuch as hydrogen, water, a hydroxyl group, or hydride can be preventedfrom being adsorbed on the surface of the silicon nitride film.

The first electrode 105 can be formed in such a manner that a conductivefilm is formed over the substrate 101 by a sputtering method, a CVDmethod, or a vacuum evaporation method, a resist mask is formed over theconductive film in a photolithography step, and the conductive film isetched using the resist mask. Alternatively, the first electrode 105 canbe formed by a printing method or an inkjet method without using aphotolithography step, so that the number of steps can be reduced. Notethat end portions of the first electrode 105 preferably have a taperedshape, so that the coverage with a gate insulating film to be formedlater improves. When the angle formed between the end portion of thefirst electrode 105 and the insulating film 103 is 30° to 60°(preferably, 40° to 50°), the coverage with the gate insulating film tobe formed later can be improved.

In this embodiment, as the conductive film for forming the firstelectrode 105, a titanium film is formed to have a thickness of 50 nm bya sputtering method, an aluminum film is formed to have a thickness of100 nm, and a titanium film is formed to have a thickness of 50 nm.Next, etching is performed using the resist mask formed in thephotolithography step, whereby the first electrode 105 having an islandshape is formed.

Next, as illustrated in FIG. 7B, the oxide semiconductor film 107 andthe second electrode 109 are formed over the first electrode 105. Theoxide semiconductor film 107 functions as a channel formation region ofthe thin film transistor, and the second electrode 109 functions as theother of the source electrode and the drain electrode of the thin filmtransistor.

Here, a method for manufacturing the oxide semiconductor film 107 andthe second electrode 109 will be described.

An oxide semiconductor film is formed by a sputtering method over thesubstrate 101 and the first electrode 105. Next, a conductive film isformed over the oxide semiconductor film.

As pretreatment, it is preferable that the substrate 101 provided withthe first electrode 105 be preheated in a preheating chamber of asputtering apparatus and impurities such as hydrogen, water, a hydroxylgroup, or hydride adsorbed on the substrate 101 be eliminated andremoved so that hydrogen is contained in the oxide semiconductor film107 as little as possible. Note that a cryopump is preferable for anevacuation unit provided in the preheating chamber. Note that thispreheating treatment can be omitted. In addition, this preheating may beperformed on the substrate 101 before the formation of the gateinsulating film 111, or may be performed on the substrate 101 before theformation of the third electrode 113 and the third electrode 115.

Note that before the oxide semiconductor film is formed by a sputteringmethod, reverse sputtering in which plasma is generated with an argongas introduced is preferably performed to remove dust attached to or anoxide film formed on the surface of the first electrode 105, so thatresistance at the interface between the first electrode 105 and theoxide semiconductor film can be reduced. The reverse sputtering refersto a method in which, without application of voltage to a target side, ahigh-frequency power source is used for application of voltage to asubstrate side in an argon atmosphere to generate plasma in the vicinityof the substrate and modify a surface. Note that instead of an argonatmosphere, a nitrogen atmosphere, a helium atmosphere, or the like maybe used.

In this embodiment, the oxide semiconductor film is formed by asputtering method with the use of an In—Ga—Zn—O-based metal oxidetarget. Alternatively, the oxide semiconductor film can be formed by asputtering method in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere of a rare gas (typically, argon) andoxygen. When a sputtering method is employed, a target containing SiO₂at 2 wt % to 10 wt % may be used.

As a sputtering gas used for forming the oxide semiconductor film, ahigh purity gas is preferably used in which impurities such as hydrogen,water, a hydroxyl group, or hydride are reduced to a concentration of 1ppm or lower (preferably, 10 ppb or lower). Note that the sputtering gasmeans a gas which is introduced into a treatment chamber wheresputtering is performed.

As a target used to form the oxide semiconductor film by a sputteringmethod, a target of a metal oxide containing zinc oxide as a maincomponent can be used. As another example of a target of a metal oxide,a metal oxide target containing In, Ga, and Zn (in a composition ratio,In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio], In:Ga:Zn=1:1:0.5 [molar ratio]) canbe used. Alternatively, as a metal oxide target containing In, Ga, andZn, a target having a composition ratio of In:Ga:Zn=1:1:1 [molar ratio]or In:Ga:Zn=1:1:2 [molar ratio] can be used. The filling rate of themetal oxide target is 90% to 100%, preferably 95% to 99.9%. An oxidesemiconductor film formed using the metal oxide target with high fillingrate as described above is dense.

The oxide semiconductor film is formed over the substrate 101 in such amanner that a sputtering gas from which hydrogen, water, a hydroxylgroup, hydride, or the like is removed is introduced into the treatmentchamber and a metal oxide is used as a target while the substrate isheld in the treatment chamber in a reduced pressure state and moistureremaining in the treatment chamber is removed. It is preferable to usean entrapment vacuum pump in order to remove hydrogen, water, a hydroxylgroup, hydride, or the like remaining in the treatment chamber. Acryopump, an ion pump, or a titanium sublimation pump is preferablyused, for example. Further, as an evacuation unit, a cold trap may beadded to a turbo pump. For example, hydrogen, water, a hydroxyl group,hydride, or the like (more preferably, also a compound containing acarbon atom) are removed from the treatment chamber which is evacuatedusing a cryopump; therefore, the concentration of impurities containedin the oxide semiconductor film can be reduced. The oxide semiconductorfilm may be formed while the substrate is heated.

In this embodiment, as an example of a film formation condition of theoxide semiconductor film, the following conditions are applied: thesubstrate temperature is room temperature, the distance between thesubstrate and the target is 110 mm; the pressure is 0.4 Pa; the directcurrent (DC) power source is 0.5 kW; and the atmosphere contains oxygenand argon (oxygen flow rate of 15 sccm, argon flow rate of 30 sccm).Note that a pulsed direct current (DC) power source is preferablebecause powder substances (also referred to as particles or dust)generated in film formation can be reduced and the film thickness can beuniform. The oxide semiconductor film preferably has a thickness of 30nm to 3000 nm. Note that the appropriate thickness of the oxidesemiconductor film differs depending on the material to be used;therefore, the thickness may be determined as appropriate in accordancewith the material.

Note that the sputtering method and sputtering apparatus that are usedfor forming the insulating film 103 can be used as appropriate as asputtering method and a sputtering apparatus for forming the oxidesemiconductor film.

The conductive film for forming the second electrode 109 can be formedusing the material and the method which are used for the first electrode105, as appropriate. Here, as the conductive film for forming the secondelectrode 109, a 50-nm-thick titanium film, a 100-nm-thick aluminumfilm, and a 50-nm-thick titanium film are stacked in that order.

Next, a resist mask is formed over the conductive film in aphotolithography step, the conductive film for forming the secondelectrode 109 and the oxide semiconductor film for forming the oxidesemiconductor film 107 are etched using the resist mask, whereby thesecond electrode 109 and the oxide semiconductor film 107 having islandshapes are formed. Instead of the resist mask formed in thephotolithography step, a resist mask can be formed using an inkjetmethod, so that the number of steps can be reduced. When the angleformed between the first electrode 105 and the end portions of thesecond electrode 109 and the oxide semiconductor film 107 is 30° to 60°(preferably, 40° to 50°) because of the etching, the coverage with agate insulating film to be formed later can be improved.

Note that the etching of the conductive film and the oxide semiconductorfilm here may be performed using either dry etching or wet etching, orusing both dry etching and wet etching. In order to form the oxidesemiconductor film 107 and the second electrode 109 each having adesired shape, an etching condition (etchant, etching time, temperature,or the like) is adjusted as appropriate in accordance with a material.

When the etching rate of each of the conductive film for forming thesecond electrode 109 and the oxide semiconductor film is different fromthat of the first electrode 105, a condition is selected such that theetching rate of the first electrode 105 is low and the etching rate ofeach of the conductive film for forming the second electrode 109 and theoxide semiconductor film is high. Alternatively, a condition is selectedsuch that the etching rate of the oxide semiconductor film is low andthe etching rate of the conductive film for forming the second electrode109 is high, and the conductive film for forming the second electrode109 is etched; then, a condition is selected such that the etching rateof the first electrode 105 is low and the etching rate of the oxidesemiconductor film is high.

As an etchant used for wet etching of the oxide semiconductor film, amixed solution of phosphoric acid, acetic acid, and nitric acid, anammonia hydrogen peroxide mixture (a 31 wt % hydrogen peroxide solution:28 wt % ammonia water:water=5:2:2), or the like can be used. Inaddition, ITO-07N (produced by KANTO CHEMICAL CO., INC.) may also beused.

The etchant after the wet etching is removed together with the etchedmaterials by cleaning. The waste liquid containing the etchant and thematerial etched off may be purified and the material may be reused. Whena material such as indium contained in the oxide semiconductor film iscollected from the waste liquid after the etching and reused, theresources can be efficiently used and the cost can be reduced.

As an etching gas used for dry etching of the oxide semiconductor film,a gas containing chlorine (a chlorine-based gas such as chlorine (Cl₂),boron trichloride (BCl₃), silicon tetrachloride (SiCl₄), or carbontetrachloride (CCl₄)) is preferably used.

Alternatively, a gas containing fluorine (a fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)), hydrogen bromide (HBr),oxygen (O₂), any of these gases to which a rare gas such as helium (He)or argon (Ar) is added, or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the film into a desired shape, the etchingconditions (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

In this embodiment, the conductive film for forming the second electrode109 is etched using an ammonia hydrogen peroxide mixture (a mixture ofammonia, water, and hydrogen peroxide water) as an etchant, and then theoxide semiconductor film is etched using a mixed solution of phosphoricacid, acetic acid, and nitric acid, whereby the oxide semiconductor film107 having an island shape is formed.

Next, in this embodiment, first heat treatment is performed. The firstheat treatment is performed at a temperature higher than or equal to400° C. and lower than or equal to 750° C., preferably, higher than orequal to 400° C. and lower than a strain point of the substrate. Here,the substrate is introduced into an electric furnace which is one ofheat treatment apparatuses, and heat treatment is performed on the oxidesemiconductor film in an inert gas atmosphere, such as a nitrogenatmosphere or a rare gas atmosphere, at 450° C. for one hour, and thenthe oxide semiconductor film is not exposed to air. Accordingly,hydrogen, water, a hydroxyl group, hydride, or the like can be preventedfrom being mixed into the oxide semiconductor film, hydrogenconcentration is reduced, and the oxide semiconductor film is purified,whereby an i-type oxide semiconductor film or a substantially i-typeoxide semiconductor film can be obtained. That is, at least one ofdehydration and dehydrogenation of the oxide semiconductor film 107 canbe performed by this first heat treatment.

Note that it is preferable that in the first heat treatment, hydrogen,water, a hydroxyl group, hydride, or the like be not contained innitrogen or a rare gas such as helium, neon, or argon. Alternatively,the purity of nitrogen or a rare gas such as helium, neon, or argonintroduced into a heat treatment apparatus is preferably 6N (99.9999%)or higher, more preferably 7N (99.99999%) or higher (that is, theconcentration of the impurities is 1 ppm or lower, preferably 0.1 ppm orlower).

Depending on the conditions of the first heat treatment or a materialfor the oxide semiconductor film, the oxide semiconductor film may becrystallized and changed to a microcrystalline film or a polycrystallinefilm in some cases. For instance, the oxide semiconductor film may becrystallized to be a microcrystalline oxide semiconductor film having adegree of crystallization of 90% or more, or 80% or more. Further,depending on the conditions of the first heat treatment and the materialof the oxide semiconductor film, the oxide semiconductor film may becomean amorphous oxide semiconductor film containing no crystallinecomponent. The oxide semiconductor film may become an oxidesemiconductor film in which a microcrystalline portion (with a graindiameter of 1 nm to 20 nm (typically, 2 nm to 4 nm) is mixed into theamorphous oxide semiconductor film.

Alternatively, the first heat treatment of the oxide semiconductor filmmay be performed on the oxide semiconductor film before the oxidesemiconductor film having an island shape is formed. In that case, thesubstrate is taken out from the heating apparatus after the first heattreatment, and then a photolithography step is performed.

Note that the heat treatment which has an effect of dehydration ordehydrogenation on the oxide semiconductor film may be performed afterthe oxide semiconductor film is formed, after the conductive film forforming the second electrode is stacked over the oxide semiconductorfilm, after the gate insulating film is formed over the first electrode,the oxide semiconductor film, and the second electrode, or after thegate electrode is formed.

Next, as illustrated in FIG. 7C, the gate insulating film 111 is formedover the first electrode 105, the oxide semiconductor film 107, and thesecond electrode 109.

The i-type oxide semiconductor film (the purified oxide semiconductorfilm whose hydrogen concentration is reduced) or the substantiallyi-type oxide semiconductor film obtained by the removal of impurities isextremely sensitive to an interface state and interface charge;therefore, the interface between the oxide semiconductor film and thegate insulating film 111 is important. Accordingly, the gate insulatingfilm 111 which is in contact with the purified oxide semiconductor filmneeds to have high quality.

For example, a high-quality insulating film which is dense and which hashigh withstand voltage can be formed by a high density plasma CVD methodusing microwaves (2.45 GHz), which is preferable. This is because whenthe purified oxide semiconductor film whose hydrogen concentration isreduced and the high-quality gate insulating film are close to eachother, the interface state can be reduced and the interfacecharacteristics can be made favorable.

Needless to say, other film formation methods, such as a sputteringmethod or a plasma CVD method, can be applied as long as a high-qualityinsulating film can be formed as the gate insulating film. A gateinsulating film whose film quality is improved, or an insulating filmwhose characteristics of an interface with the oxide semiconductor filmare improved, by the heat treatment after the gate insulating film isformed may be used. In any case, any insulating film that has a reducedinterface state density and can form a favorable interface with theoxide semiconductor as well as having a favorable film quality as a gateinsulating film can be used.

Further, when an oxide semiconductor film containing impurities issubjected to a gate bias-temperature stress test (BT test) at 85° C., ata voltage applied to the gate of 2×10⁶ V/cm, for 12 hours, a bondbetween the impurity and a main component of the oxide semiconductorfilm is cleaved by a high electric field (B: bias) and a hightemperature (T: temperature), and a generated dangling bond inducesdrift of threshold voltage (V_(th)).

In contrast, the present invention makes it possible to obtain a thinfilm transistor which is stable to a BT test by removing impurities inan oxide semiconductor film, especially hydrogen, water, and the like asmuch as possible to obtain a favorable characteristic of an interfacebetween the oxide semiconductor and a gate insulating film as describedabove.

When the gate insulating film 111 is formed by a sputtering method, thehydrogen concentration in the gate insulating film 111 can be reduced.When a silicon oxide film is formed by a sputtering method, silicon orquartz is used as a target and oxygen or a mixed gas of oxygen and argonis used as a sputtering gas.

The gate insulating film 111 can have a structure in which a siliconoxide film and a silicon nitride film are stacked in that order over thefirst electrode 105, the oxide semiconductor film 107, and the secondelectrode 109. For example, a silicon oxide film (SiO_(x) (x>0)) havinga thickness of 5 nm to 300 nm is formed as a first gate insulating film,and a silicon nitride film (SiN_(y) (y>0)) having a thickness of 50 nmto 200 nm is stacked as a second gate insulating film over the firstgate insulating film by a sputtering method, so that a gate insulatingfilm having a thickness of 100 nm may be formed. In this embodiment, asilicon oxide film having a thickness of 100 nm is formed by an RFsputtering method under the following conditions: the pressure is 0.4Pa; the high-frequency power is 1.5 kW; and the atmosphere containsoxygen and argon (the flow ratio of oxygen to argon is 1:1 (each flowrate is 25 sccm)).

Next, second heat treatment may be performed in an inert gas atmosphereor an oxygen gas atmosphere (preferably, at 200° C. to 400° C., forexample, 250° C. to 350° C.). Note that the second heat treatment may beperformed after the formation of at least one of the third electrode113, the third electrode 115, the insulating film 117, and the wirings125 and 131, which is performed later. Hydrogen or moisture contained inthe oxide semiconductor film can be diffused into the gate insulatingfilm by the heat treatment.

Then, the third electrode 113 and the third electrode 115 which functionas a gate electrode are formed over the gate insulating film 111.

The third electrode 113 and the third electrode 115 can be formed insuch a manner that a conductive film for forming the third electrode 113and the third electrode 115 is formed over the gate insulating film 111by a sputtering method, a CVD method, or a vacuum evaporation method, aresist mask is formed in a photolithography step over the conductivefilm, and the conductive film is etched using the resist mask.

In this embodiment, after a titanium film having a thickness of 150 nmis formed by a sputtering method, etching is performed using a resistmask formed in a photolithography step, so that the third electrode 113and the third electrode 115 are formed.

Through the above process, the thin film transistor 133 having thepurified oxide semiconductor film 107 whose hydrogen concentration isreduced can be formed.

Next, as illustrated in FIG. 7D, after the insulating film 117 is formedover the gate insulating film 111, the third electrode 113, and thethird electrode 115, a contact hole 119, a contact hole 121, a contacthole 123, and a contact hole are formed.

The insulating film 117 is formed using an oxide insulating film such asa silicon oxide film, a silicon oxynitride film, an aluminum oxide film,or an aluminum oxynitride film, or a nitride insulating film such as asilicon nitride film, a silicon nitride oxide film, an aluminum nitridefilm, or an aluminum nitride oxide film. Alternatively, an oxideinsulating film and a nitride insulating film can be stacked.

The insulating film 117 is formed by a sputtering method, a CVD method,or the like. Note that when the insulating film 117 is formed by asputtering method, the substrate 101 may be heated to a temperature of100° C. to 400° C., a sputtering gas in which hydrogen, water, ahydroxyl group, hydride, or the like is removed and which contains highpurity nitrogen may be introduced, and an insulating film may be formedusing a silicon target. Also in this case, an insulating film ispreferably formed while hydrogen, water, a hydroxyl group, hydride, orthe like remaining in the treatment chamber is removed.

Note that after the insulating film 117 is formed, heat treatment may beperformed in the air at a temperature of 100° C. to 200° C. for 1 hourto 30 hours. A normally-off thin film transistor can be obtained by thisheat treatment. Therefore, reliability of a semiconductor device can beimproved.

A resist mask is formed in a photolithography step, and parts of thegate insulating film 111 and the insulating film 117 are removed byselective etching, whereby the contact hole, the contact hole 119, thecontact hole 121, and the contact hole 123 which reach the firstelectrode 105, the third electrode 113, the third electrode 115, and thesecond electrode 109 are formed.

Next, after a conductive film is formed over the gate insulating film111, the contact hole 119, the contact hole 121, and the contact hole123, etching is performed using a resist mask formed in aphotolithography step, whereby the wiring 125 and the wiring 131 areformed. Note that the resist mask may be formed by an inkjet method. Nophotomask is used when a resist mask is formed by an inkjet method;therefore, production cost can be reduced.

The wiring 125 and the wiring 131 can be formed in a manner similar tothat of the first electrode 105.

Note that a planarization insulating film for planarization may beprovided between the third electrodes 113 and 115 and the wirings 125and 131. An organic material having heat resistance, such as polyimide,acrylic, benzocyclobutene, polyamide, or epoxy can be used as typicalexamples of the planarization insulating film. Other than such organicmaterials, it is also possible to use a low-dielectric constant material(a low-k material), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like. Note that theplanarization insulating film may be formed by stacking a plurality ofinsulating films formed from these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include an organic group (e.g.,an alkyl group or an aryl group) or a fluoro group as a substituent.Moreover, the organic group may include a fluoro group.

There is no particular limitation on the method for forming theplanarization insulating film. The planarization insulating film can beformed, depending on the material, by a method such as a sputteringmethod, an SOG method, a spin coating method, a dipping method, a spraycoating method, or a droplet discharge method (e.g., an inkjet method,screen printing, or offset printing), or a tool such as a doctor knife,a roll coater, a curtain coater, or a knife coater.

Through the above process, the hydrogen concentration in the oxidesemiconductor film can be reduced, and the oxide semiconductor film canbe purified. Accordingly, the oxide semiconductor film can bestabilized. In addition, an oxide semiconductor film which has anextremely small number of minority carriers and a wide band gap can beformed by heat treatment at a temperature of lower than or equal to theglass transition temperature. As a result, a thin film transistor can beformed using a large-area substrate; thus, the mass productivity can beimproved. In addition, with the use of the purified oxide semiconductorfilm whose hydrogen concentration is reduced, it is possible tomanufacture a thin film transistor which is suitable for higherdefinition, has high operation speed, and is capable of conducting alarge amount of current when turned on and almost no current when turnedoff.

By connecting a source or a drain of a thin film transistor to a gatethereof as described above, a diode in which reverse current is verysmall can be obtained. Therefore, a diode which is resistant to abreakdown (i.e., has high withstand voltage) can be manufactured.

Note that in order to eliminate impurities such as hydrogen, water, ahydroxyl group, or hydride (also referred to as a hydrogen compound)which may exist in the oxide semiconductor film or at the interfacebetween the oxide semiconductor film and an insulating film that isprovided in contact with the oxide semiconductor film, a halogen element(e.g., fluorine or chlorine) may be contained in the insulating filmthat is provided in contact with the oxide semiconductor film, or ahalogen element may be contained in an oxide semiconductor film byplasma treatment in a gas atmosphere containing a halogen element in astate where the oxide semiconductor film is exposed. When the insulatingfilm contains a halogen element, the halogen element concentration inthe insulating film may be approximately 5×10¹⁸ atoms/cm³ to 1×10²⁰atoms/cm³.

As described above, in the case where a halogen element is contained inthe oxide semiconductor film or at the interface between the oxidesemiconductor film and the insulating film that is in contact with theoxide semiconductor film and the insulating film that is provided incontact with the oxide semiconductor film is an oxide insulating film, aside of the oxide insulating film which is not in contact with the oxidesemiconductor film is preferably covered with a nitrogen-basedinsulating film. That is, a silicon nitride film or the like may beprovided on and in contact with the oxide insulating film that is incontact with the oxide semiconductor film. With such a structure,impurities such as hydrogen, water, a hydroxyl group, or hydride can beprevented from entering the oxide insulating film.

Note that the diodes illustrated in FIGS. 2A and 2B, FIGS. 3A and 3B,FIGS. 4A and 4B, FIGS. 5A and 5B, and FIGS. 6A and 6B can also be formedin a similar manner.

This embodiment can be implemented in an appropriate combination withany of structures described in other embodiments.

Embodiment 5

In this embodiment, a diode-connected thin film transistor including anoxide semiconductor film which is different from that described inEmbodiment 4, and a manufacturing method thereof, will be described withreference to FIGS. 7A and 7B and FIGS. 8A and 8B.

In a manner similar to that in Embodiment 4, as illustrated in FIG. 7A,the insulating film 103 and the first electrode 105 are formed over thesubstrate 101. Next, as illustrated in FIG. 7B, the oxide semiconductorfilm 107 and the second electrode 109 are formed over the firstelectrode 105.

Next, first heat treatment is performed. The first heat treatment inthis embodiment is different from the first heat treatment in the aboveembodiment. The heat treatment makes it possible to form an oxidesemiconductor film 151 in which crystal grains are formed at the surfaceas illustrated in FIG. 8A. In this embodiment, the first heat treatmentis performed with an apparatus for heating an object to be processed byat least one of thermal conduction and thermal radiation from a heatersuch as a resistance heater. Here, the temperature of the heat treatmentis 500° C. to 700° C., preferably 650° C. to 700° C. Note that, althoughthere is no requirement for the upper limit of the heat treatmenttemperature from the essential part of the invention, the upper limit ofthe heat treatment temperature needs to be within the allowabletemperature limit of the substrate 101. In addition, the length of timeof the heat treatment is preferably 1 minute to 10 minutes. When RTAtreatment is employed for the first heat treatment, the heat treatmentcan be performed in a short time; thus, adverse effects of heat on thesubstrate 101 can be reduced. In other words, the upper limit of theheat treatment temperature can be raised in this case as compared withthe case where heat treatment is performed for a long time. In addition,the crystal grains having predetermined structures can be selectivelyformed in the vicinity of the surface of the oxide semiconductor film.

As examples of the heat treatment apparatus that can be used in thisembodiment, rapid thermal annealing (RTA) apparatuses such as a gasrapid thermal annealing (GRTA) apparatus and a lamp rapid thermalannealing (LRTA) apparatus, and the like are given. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas,an inert gas which does not react with an object to be processed by heattreatment, such as nitrogen or a rare gas like argon, is used.

For example, as the first heat treatment, GRTA may be performed in whichthe substrate is moved into an atmosphere of an inert gas such asnitrogen or a rare gas which has been heated to a temperature as high as650° C. to 700° C., and the substrate is heated for several minutes andmoved out of the inert gas which has been heated to a high temperature.GRTA enables high-temperature heat treatment to be performed in a shorttime.

Note that in the first heat treatment, it is preferable that hydrogen,water, a hydroxyl group, hydride, or the like be not contained innitrogen or a rare gas such as helium, neon, or argon. Alternatively,the purity of nitrogen or a rare gas such as helium, neon, or argon thatis introduced into the heat treatment apparatus is preferably 6N(99.9999%) or higher, more preferably 7N (99.99999%) or higher (that is,the impurity concentration is 1 ppm or lower, preferably 0.1 ppm orlower).

Note that the above heat treatment may be performed at any timing aslong as it is performed after the oxide semiconductor film 107 isformed; however, in order to promote dehydration or dehydrogenation, theheat treatment is preferably performed before other components areformed on a surface of the oxide semiconductor film 107. In addition,the heat treatment may be performed plural times instead of once.

FIG. 8B is an enlarged view of a dashed line portion 153 in FIG. 8A.

The oxide semiconductor film 151 includes an amorphous region 155 thatmainly contains an amorphous oxide semiconductor and crystal grains 157that are formed in the surface of the oxide semiconductor film 151.Further, the crystal grains 157 are formed in a region extending fromthe surface to a distance (depth) of 20 nm or less (in the vicinity ofthe surface). Note that the location where the crystal grains 157 areformed is not limited to the above in the case where the thickness ofthe oxide semiconductor film 151 is large. For example, in the casewhere the oxide semiconductor film 151 has a thickness of 200 nm ormore, the “vicinity of a surface (surface vicinity)” means a regionextending from the surface to a distance (depth) that is 10% or less ofthe thickness of the oxide semiconductor film.

Here, the amorphous region 155 mainly contains an amorphous oxidesemiconductor film. Note that the word “mainly” means, for example, astate where one occupies 50% or more of a region. In this case, it meansa state where the amorphous oxide semiconductor film occupies 50% ormore at volume % (or weight %) of the amorphous region 155. In otherwords, the amorphous region in some cases includes crystals of an oxidesemiconductor film other than an amorphous oxide semiconductor film, andthe percentage of the content thereof is preferably less than 50% atvolume % (or weight %). However, the percentage of the content is notlimited to the above range.

In the case where an In—Ga—Zn—O-based oxide semiconductor is used as amaterial for the oxide semiconductor film, the composition of the aboveamorphous region 155 is preferably set so that the Zn content (atomic %)is less than the In or Ga content (atomic %) for the reason that suchcomposition makes it easy for the crystal grains 157 which havepredetermined composition to be formed.

After that, a gate insulating film and a third electrode that functionsas a gate electrode are formed in a manner similar to that of Embodiment4 to complete the thin film transistor.

The vicinity of the surface of the oxide semiconductor film 151, whichis in contact with the gate insulating film, serves as a channel. Thecrystal grains are included in the region that serves as a channel,whereby the resistance between a source, the channel, and a drain isreduced and carrier mobility is increased. Thus, the field-effectmobility of the thin film transistor in which the oxide semiconductorfilm 151 is included is increased, which leads to favorable electriccharacteristics of the thin film transistor.

Further, the crystal grains 157 are more stable than the amorphousregion 155; thus, when the crystal grains 157 are included in thevicinity of the surface of the oxide semiconductor film 151, entry ofimpurities (e.g., hydrogen, water, a hydroxyl group, or hydride) intothe amorphous region 155 can be reduced. Thus, the reliability of theoxide semiconductor film 151 can be improved.

Through the above process, the concentration of hydrogen in the oxidesemiconductor film can be reduced and the oxide semiconductor film canbe purified. Thus, stabilization of the oxide semiconductor film can beachieved. In addition, heat treatment at a temperature of lower than orequal to the glass transition temperature makes it possible to form anoxide semiconductor film with a wide band gap in which the number ofminority carriers is extremely small. Thus, thin film transistors can bemanufactured using a large-area substrate; thus, the mass productivitycan be improved. Further, with the use of the purified oxidesemiconductor film whose hydrogen concentration is reduced, it ispossible to manufacture a thin film transistor which is suitable forhigher definition, has high operation speed, and is capable ofconducting a large amount of current when turned on and almost nocurrent when turned off.

By connecting a source or a drain of a thin film transistor to a gatethereof as described above, a diode in which reverse current is verysmall can be obtained. Therefore, a diode which is resistant to abreakdown (i.e., has high withstand voltage) can be manufactured.

This embodiment can be implemented in an appropriate combination withany of structures described in other embodiments.

Embodiment 6

In this embodiment, a manufacturing process of the diode-connected thinfilm transistor illustrated in FIGS. 1A and 1B, which is different fromthose described in Embodiments 4 and 5, will be described with referenceto FIGS. 7A to 7E.

In a manner similar to that of Embodiment 4, as illustrated in FIG. 7A,the first electrode 105 is formed over the substrate 101.

Next, as illustrated in FIG. 7B, the oxide semiconductor film 107 andthe second electrode 109 are formed over the first electrode 105.

Note that before the oxide semiconductor film is formed by a sputteringmethod, reverse sputtering in which plasma is generated with an argongas introduced is preferably performed so that dust attached to or anoxide film formed on the surface of the first electrode 105 is removed,in which case the resistance at the interface between the firstelectrode 105 and the oxide semiconductor film can be reduced. Note thatinstead of an argon atmosphere, a nitrogen atmosphere, a heliumatmosphere, or the like may be used.

The oxide semiconductor film is formed over the substrate 101 and thefirst electrode 105 by a sputtering method. Then, a conductive film isformed over the oxide semiconductor film.

In this embodiment, the oxide semiconductor film is formed by asputtering method using an In—Ga—Zn—O-based metal oxide target. In thisembodiment, the substrate is held in a treatment chamber in a reducedpressure state, and the substrate is heated to room temperature or atemperature lower than 400° C. Then, the oxide semiconductor film isformed over the substrate 101 and the first electrode 105 in such amanner that a sputtering gas from which hydrogen, water, a hydroxylgroup, hydride, or the like is removed is introduced and a metal oxideis used as a target while hydrogen, water, a hydroxyl group, hydride, orthe like remaining in the treatment chamber is removed. An entrapmentvacuum pump is preferably used for removing hydrogen, water, a hydroxylgroup, hydride, or the like remaining in the treatment chamber. Forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. An evacuation unit may be a turbo pump provided with acold trap. From the treatment chamber evacuated with a cryopump, forexample, hydrogen, water, a hydroxyl group, hydride (preferably, also acompound containing a carbon atom), or the like is eliminated; thus, theconcentration of impurities contained in the oxide semiconductor filmformed in the treatment chamber can be reduced. Further, sputteringformation is performed while hydrogen, water, a hydroxyl group, hydride,or the like remaining in the treatment chamber is removed with acryopump, whereby an oxide semiconductor film in which impurities suchas hydrogen atoms and water are reduced can be formed even at asubstrate temperature of room temperature to a temperature lower than400° C.

In this embodiment, film formation conditions where the distance betweenthe substrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power is 0.5 kW, and the atmosphere is an oxygenatmosphere (the proportion of oxygen flow is 100%) are employed. Notethat a pulsed direct current (DC) power source is preferable becausepowder substances (also referred to as particles or dust) generated infilm formation can be reduced and the film thickness can be uniform. Theoxide semiconductor film preferably has a thickness of 30 nm to 3000 nm.Note that the appropriate thickness of the oxide semiconductor filmdiffers depending on the material to be used; therefore, the thicknessmay be determined as appropriate in accordance with the material.

Note that the sputtering method and sputtering apparatus that are usedfor forming the insulating film 103 can be used as appropriate as asputtering method and a sputtering apparatus for forming the oxidesemiconductor film.

Next, a conductive film for forming the second electrode 109 is formedusing the material and method that are used for forming the firstelectrode 105.

Next, in a manner similar to that of Embodiment 4, the conductive filmfor forming the second electrode 109 and the oxide semiconductor filmfor forming the oxide semiconductor film 107 are etched so that thesecond electrode 109 and the oxide semiconductor film 107 having anisland shape are formed. The etching conditions (such as an etchant,etching time, and temperature) are adjusted as appropriate in accordancewith the material in order to form the oxide semiconductor film 107 andthe second electrode 109 having desired shapes.

Next, as illustrated in FIG. 7C, in a manner similar to that ofEmbodiment 4, the gate insulating film 111 is formed over the firstelectrode 105, the oxide semiconductor film 107, and the secondelectrode 109. As the gate insulating film 111, a gate insulating filmthat has a favorable characteristic of an interface between the gateinsulating film 111 and the oxide semiconductor film 107 is preferable.The gate insulating film 111 is preferably formed by a high densityplasma CVD method using microwaves (2.45 GHz), in which case the gateinsulating film 111 can be dense and can have high withstand voltage andhigh quality. Another method such as a sputtering method or a plasma CVDmethod can be employed as long as the method enables a good-qualityinsulating film to be formed as the gate insulating film.

Note that before the gate insulating film 111 is formed, reversesputtering is preferably performed so that resist residues and the likeattached to at least a surface of the oxide semiconductor film 107 canbe removed.

Further, before the gate insulating film 111 is formed, hydrogen, water,a hydroxyl group, hydride, or the like attached to an exposed surface ofthe oxide semiconductor film may be removed by plasma treatment using agas such as N₂O, N₂, or Ar. Alternatively, plasma treatment may beperformed using a mixed gas of oxygen and argon. In the case whereplasma treatment is performed, the gate insulating film 111 which is tobe in contact with part of the oxide semiconductor film is preferablyformed without being exposed to air.

Further, it is preferable that the substrate 101 over which componentsup to and including the first electrode 105 to the second electrode 109are formed be preheated in a preheating chamber in a sputteringapparatus as pretreatment to eliminate and remove hydrogen, water, ahydroxyl group, hydride, or the like adsorbed on the substrate 101 sothat hydrogen, water, a hydroxyl group, hydride, or the like iscontained as little as possible in the gate insulating film 111.Alternatively, it is preferable that the substrate 101 be preheated in apreheating chamber in a sputtering apparatus to eliminate and removeimpurities such as hydrogen, water, a hydroxyl group, hydride, or thelike adsorbed on the substrate 101 after the gate insulating film 111 isformed. Note that the temperature of the preheating is 100° C. to 400°C., preferably 150° C. to 300° C. A cryopump is preferable as anevacuation unit provided in the preheating chamber. Note that thispreheating treatment can be omitted.

The gate insulating film 111 may have a structure in which a siliconoxide film and a silicon nitride film are stacked in that order over thefirst electrode 105, the oxide semiconductor film 107, and the secondelectrode 109. For example, a silicon oxide film (SiO_(x) (x>0)) havinga thickness of 5 nm to 300 nm is formed as a first gate insulating filmby a sputtering method and a silicon nitride film (SiN_(y) (y>0)) havinga thickness of 50 nm to 200 nm is stacked as a second gate insulatingfilm over the first gate insulating film, whereby the gate insulatingfilm 111 is formed.

Next, as illustrated in FIG. 7C, in a manner similar to that ofEmbodiment 4, the third electrode 113 and the third electrode 115 thatfunction as a gate electrode are formed over the gate insulating film111.

Through the above process, the thin film transistor 133 including theoxide semiconductor film 107 in which the hydrogen concentration isreduced can be manufactured.

Hydrogen, water, a hydroxyl group, hydride, or the like remaining in areaction atmosphere is removed in forming the oxide semiconductor filmas described above, whereby the concentration of hydrogen in the oxidesemiconductor film can be reduced. Thus, stabilization of the oxidesemiconductor film can be achieved.

Next, as illustrated in FIG. 7D, in a manner similar to that ofEmbodiment 4, the contact hole 119, the contact hole 121, and thecontact hole 123 are formed after the insulating film 117 is formed overthe gate insulating film 111, the third electrode 113, and the thirdelectrode 115.

Next, as illustrated in FIG. 7E, in a manner similar to that ofEmbodiment 4, the wiring 125 and the wiring 131 are formed.

In a manner similar to that of Embodiment 4, after the formation of theinsulating film 117, heat treatment may be further performed at atemperature of 100° C. to 200° C. in air for 1 hour to 30 hours. Anormally-off thin film transistor can be obtained by this heattreatment. Therefore, the reliability of a semiconductor device can beimproved.

Note that a planarization insulating film for planarization may beprovided between the third electrodes 113 and 115 and the wirings 125and 131.

Hydrogen, water, a hydroxyl group, hydride, or the like remaining in areaction atmosphere is removed in forming the oxide semiconductor filmas described above, whereby the concentration of hydrogen in the oxidesemiconductor film can be reduced and the oxide semiconductor film canbe purified. Thus, stabilization of the oxide semiconductor film can beachieved. In addition, an oxide semiconductor film which has anextremely small number of minority carriers and a wide band gap can beformed by heat treatment at a temperature of lower than or equal to theglass transition temperature. As a result, a thin film transistor can beformed using a large-area substrate; thus, the mass productivity can beimproved. In addition, with the use of the purified oxide semiconductorfilm whose hydrogen concentration is reduced, it is possible tomanufacture a thin film transistor which is suitable for higherdefinition, has high operation speed, and is capable of conducting alarge amount of current when turned on and almost no current when turnedoff.

By connecting a source or a drain of a thin film transistor to a gatethereof as described above, a diode in which reverse current is verysmall can be obtained. Therefore, a diode which is resistant to abreakdown (i.e., has high withstand voltage) can be manufactured.

This embodiment can be implemented in an appropriate combination withany of structures described in other embodiments.

Embodiment 7

The diode which is described in the above embodiment can be applied to asemiconductor device. As an example of the semiconductor device, adisplay device can be given.

The structure of a display device which is one embodiment of the presentinvention will be described with reference to FIG. 9. FIG. 9 is a topview of a substrate 200 of the display device. A pixel portion 201 isformed over the substrate 200. In addition, an input terminal 202 and aninput terminal 203 supply signals and power for displaying images to apixel circuit formed over the substrate 200.

Note that the display device which is one embodiment of the presentinvention is not limited to that illustrated in FIG. 9. That is, one ofor both a scan line driver circuit and a signal line driver circuit maybe formed over the substrate 200.

The input terminal 202 on the scan line side and the input terminal 203on the signal line side which are formed over the substrate 200 areconnected to the pixel portion 201 by wirings extended vertically andhorizontally. The wirings are connected to protection circuits 204 to207.

The pixel portion 201 and the input terminal 202 are connected by awiring 209. The protection circuit 204 is placed between the pixelportion 201 and the input terminal 202 and is connected to the wiring209. When the protection circuit 204 is provided, various semiconductorelements such as thin film transistors, which are included in the pixelportion 201, can be protected and deterioration or damage thereof can beprevented. Note that although the wiring 209 corresponds to one wiringin the drawing, all of a plurality of wirings provided in parallel withthe wiring 209 have connection relations which are similar to that ofthe wiring 209. Note that the wiring 209 functions as a scan line.

Note that on the scan line side, not only the protection circuit 204between the input terminal 202 and the pixel portion 201 but also aprotection circuit on the side of the pixel portion 201 which isopposite to the input terminal 202 may be provided (see the protectioncircuit 205 in FIG. 9).

Meanwhile, the pixel portion 201 and the input terminal 203 areconnected by a wiring 208. The protection circuit 206 is placed betweenthe pixel portion 201 and the input terminal 203 and is connected to thewiring 208. When the protection circuit 206 is provided, varioussemiconductor elements such as thin film transistors, which are includedin the pixel portion 201, can be protected and deterioration or damagethereof can be prevented. Note that although the wiring 208 correspondsto one wiring in the drawing, all of a plurality of wirings provided inparallel with the wiring 208 have connection relations which are similarto that of the wiring 208. Note that the wiring 208 functions as asignal line.

Note that on the signal line side, not only the protection circuit 206between the input terminal 203 and the pixel portion 201 but also aprotection circuit on the side of the pixel portion 201 which isopposite to the input terminal 203 may be provided (see the protectioncircuit 207 in FIG. 9).

Note that all the protection circuits 204 to 207 are not necessarilyprovided. However, it is necessary to provide at least the protectioncircuit 204. This is because when excessive current is generated in thescan line, gate insulating layers of the thin film transistors includedin the pixel portion 201 are damaged and a number of point defects canbe generated in some cases.

In addition, when not only the protection circuit 204 but also theprotection circuit 206 is provided, generation of excessive current inthe signal line can be prevented. Therefore, compared to the case whereonly the protection circuit 204 is provided, reliability is improved andyield is improved. When the protection circuit 206 is provided,breakdown due to static electricity which can be generated in a rubbingprocess or the like after forming the thin film transistors can beprevented.

Further, when the protection circuit 205 and the protection circuit 207are provided, reliability can further be improved. Moreover, yield canbe improved. The protection circuit 205 and the protection circuit 207are provided opposite to the input terminal 202 and the input terminal203, respectively. Therefore, the protection circuit 205 and theprotection circuit 207 can prevent deterioration and breakdown ofvarious semiconductor elements, which are caused in a manufacturing stepof the display device (e.g., a rubbing process in manufacturing a liquidcrystal display device).

Note that in FIG. 9, a signal line driver circuit and a scan line drivercircuit which are formed separately from the substrate 200 are mountedon the substrate 200 by a known method such as a COG method or a TABmethod. However, the present invention is not limited thereto. The scanline driver circuit and the pixel portion may be formed over thesubstrate 200, and the signal line driver circuit which is formedseparately may be mounted. Alternatively, part of the scan line drivercircuit or part of the signal line driver circuit, and the pixel portion201 may be formed over the substrate 200, and the other part of the scanline driver circuit or the other part of the signal line driver circuitmay be mounted. When part of the scan line driver circuit is providedbetween the pixel portion 201 and the input terminal 202 on the scanline side, a protection circuit may be provided between the inputterminal 202 on the scan line side and part of the scan line drivercircuit over the substrate 200, or a protection circuit may be providedbetween part of the scan line driver circuit and the pixel portion 201,or protection circuits may be provided between the input terminal 202 onthe scan line side and part of the scan line driver circuit over thesubstrate 200 and between part of the scan line driver circuit and thepixel portion 201. Alternatively, when part of the signal line drivercircuit is provided between the pixel portion 201 and the input terminal203 on the signal line side, a protection circuit may be providedbetween the input terminal 203 on the signal line side and part of thesignal line driver circuit over the substrate 200, or a protectioncircuit may be provided between part of the signal line driver circuitand the pixel portion 201, or protection circuits may be providedbetween the input terminal 203 on the signal line side and part of thesignal line driver circuit over the substrate 200 and between part ofthe signal line driver circuit and the pixel portion 201. That is, sincevarious modes are used for driver circuits, the number and position ofprotection circuits are determined in accordance with the modes of thedriver circuits.

Next, examples of a specific circuit structure of a protection circuitwhich is used as the protection circuits 204 to 207 in FIG. 9 aredescribed with reference to FIGS. 10A to 10F. Only the case where ann-channel transistor is provided is described below.

A protection circuit illustrated in FIG. 10A includes protection diodes211 to 214 each including a plurality of thin film transistors. Theprotection diode 211 includes an n-channel thin film transistor 211 aand an n-channel thin film transistor 211 b which are connected inseries. One of a source electrode and a drain electrode of the n-channelthin film transistor 211 a is connected to a gate electrode of then-channel thin film transistor 211 a and a gate electrode of then-channel thin film transistor 211 b and is kept at a potential V_(ss).The other of the source electrode and the drain electrode of then-channel thin film transistor 211 a is connected to one of a sourceelectrode and a drain electrode of the n-channel thin film transistor211 b. The other of the source electrode and the drain electrode of then-channel thin film transistor 211 b is connected to the protectiondiode 212. Further, in a manner similar to that of the protection diode211, the protection diodes 212 to 214 each include a plurality of thinfilm transistors connected in series, and one end of the plurality ofthin film transistors connected in series is connected to gateelectrodes of the plurality of thin film transistors.

Note that the number and polarity of the thin film transistors includedin the protection diodes 211 to 214 are not limited to those illustratedin FIG. 10A. For example, the protection diode 211 may include threethin film transistors connected in series.

The protection diodes 211 to 214 are sequentially connected in series,and a wiring 215 is connected to a wiring between the protection diode212 and the protection diode 213. Note that the wiring 215 is a wiringelectrically connected to a semiconductor element which is to beprotected. Note that a wiring connected to the wiring 215 is not limitedto a wiring between the protection diode 212 and the protection diode213. That is, the wiring 215 may be connected to a wiring between theprotection diode 211 and the protection diode 212, or may be connectedto a wiring between the protection diode 213 and the protection diode214.

One end of the protection diode 214 is kept at a power supply potentialV_(dd). In addition, the protection diodes 211 to 214 are connected sothat reverse bias voltage is applied to each of the protection diodes211 to 214.

A protection circuit illustrated in FIG. 10B includes a protection diode220, a protection diode 221, a capacitor 222, a capacitor 223, and aresistor 224. The resistor 224 is a resistor having two terminals; oneof the terminals is supplied with a potential V_(in) from a wiring 225,and the other is supplied with the potential V_(ss). The resistor 224 isprovided in order to set the potential of the wiring 225 to V_(ss) whenthe potential V_(in) is not supplied, and the resistance value of theresistor 224 is set sufficiently larger than the wiring resistance ofthe wiring 225. Diode-connected n-channel thin film transistors are usedfor the protection diode 220 and the protection diode 221.

Note that the protection diodes illustrated in FIGS. 10A to 10F may beconfigured with two or more thin film transistors connected in series.

Here, the case where the protection circuits illustrated in FIGS. 10A to10F are operated is described. At this time, one of source or drainelectrodes of each of the protection diodes 211, 212, 221, 230, 231,234, and 235, which is kept at the potential V_(ss), is a drainelectrode, and the other is a source electrode. One of source or drainelectrodes of each of the protection diodes 213, 214, 220, 232, 233,236, and 237, electrodes, which is kept at the potential V_(dd), is asource electrode, and the other is a drain electrode. In addition, thethreshold voltage of the thin film transistors included in theprotection diodes is denoted by V_(th).

Further, as for the protection diodes 211, 212, 221, 230, 231, 234, and235, when the potential V_(in) is higher than the potential V_(ss),reverse bias voltage is applied thereto and current does not easily flowtherethrough. Meanwhile, as for the protection diodes 213, 214, 220,232, 233, 236, and 237, when the potential V_(in) is lower than thepotential V_(dd), reverse bias voltage is applied thereto and currentdoes not easily flow therethrough.

Here, operations of the protection circuits in which a potential V_(out)is set roughly between the potential V_(ss) and the potential V_(dd) aredescribed.

First, the case where the potential V_(in) is higher than the potentialV_(dd) is described. When the potential V_(in) is higher than thepotential V_(dd), the n-channel thin film transistors are turned on whena potential difference between the gate electrodes and the sourceelectrodes of the protection diodes 213, 214, 220, 232, 233, 236, and237 is V_(gs)=V_(in)−V_(dd)>V_(th). Here, since the case where V_(in) isunusually high is assumed, the n-channel thin film transistors areturned on. At this time, the n-channel thin film transistors included inthe protection diodes 211, 212, 221, 230, 231, 234, and 235 are turnedoff. Then, the potential V_(out) becomes V_(dd) through the protectiondiodes 213, 214, 220, 232, 233, 236, and 237. Therefore, even when thepotential V_(in) is unusually higher than the potential V_(dd) due tonoise or the like, the potential V_(out) does not become higher than thepotential V_(dd).

On the other hand, when the potential V_(in) is lower than the potentialV_(ss) and a potential difference between the gate electrodes and thesource electrodes of the protection diodes 211, 212, 221, 230, 231, 234,and 235 is V_(gs)=V_(ss)−V_(in)>V_(th), the n-channel thin filmtransistors are turned on. Here, since the case where V_(in) isunusually low is assumed, the n-channel thin film transistors are turnedon. At this time, the n-channel thin film transistors included in theprotection diodes 213, 214, 220, 232, 233, 236, and 237 are turned off.Then, the potential V_(out) becomes V_(ss) through the protection diodes211, 212, 221, 230, 231, 234, and 235. Therefore, even when thepotential V_(in) is unusually lower than the potential V_(ss) due tonoise or the like, the potential V_(out) does not become lower than thepotential V_(ss). Further, the capacitor 222 and the capacitor 223reduce pulsed noise of the input potential V_(in) and relieve a steepchange in potential due to noise.

Note that when the potential V_(in) is between V_(ss)−V_(th) andV_(dd)+V_(th), all the n-channel thin film transistors included in theprotection diodes are turned off, and the potential V_(in) is input tothe potential V_(out).

When the protection circuit is provided as described above, thepotential V_(out) is kept roughly between the potential V_(ss) and thepotential V_(dd). Therefore, the potential V_(out) can be prevented fromdeviating from this range greatly. That is, the potential V_(out) can beprevented from becoming unusually high or unusually low, a circuit in asubsequent stage of the protection circuit can be prevented from beingdamaged or deteriorating, and the circuit in a subsequent stage can beprotected.

Further, as illustrated in FIG. 10B, when the protection circuitincluding the resistor 224 is provided for an input terminal, potentialsof all the wirings supplied with a signal can be kept constant (here thepotential V_(ss)) when a signal is not input. That is, when a signal isnot input, the protection circuit also functions as a short-circuit ringcapable of short-circuiting the wirings. Therefore, electrostaticbreakdown caused by a potential difference between the wirings can beprevented. In addition, since the resistance of the resistor 224 issufficiently larger than wiring resistance, a signal supplied to thewiring can be prevented from dropping to the potential V_(ss) at thetime of inputting the signal.

Here, as an example, the case is described in which n-channel thin filmtransistors having the threshold voltage V_(th)=0 are used for theprotection diode 220 and the protection diode 221 in FIG. 10B.

First, in the case of V_(in)>V_(dd), the protection diode 220 is turnedon because V_(gs)=V_(in)−V_(dd)>0. The protection diode 221 is turnedoff. Therefore, the potential of the wiring 225 becomes V_(dd), so thatV_(out)=V_(dd).

On the other hand, in the case of V_(in)<V_(ss), the protection diode220 is turned off. The protection diode 221 is turned on becauseV_(gs)=V_(ss)−V_(in)>0. Therefore, the potential of the wiring 225becomes V_(ss), so that V_(out)=V_(ss).

Even in the case of V_(in)<V_(ss) or V_(dd)<V_(in) in this manner,operations can be performed in a range of V_(ss)<V_(out)<V_(dd).Therefore, even in the case where V_(in) is too high or too low, V_(out)can be prevented from becoming too high or too low. Accordingly, forexample, even when the potential V_(in) is lower than the potentialV_(ss) due to noise or the like, the potential of the wiring 225 doesnot become extremely lower than the potential V_(ss). Further, thecapacitor 222 and the capacitor 223 reduce pulsed noise of the inputpotential V_(in) and relieve a steep change in potential.

When the protection circuit is provided as described above, thepotential of the wiring 225 is kept roughly between the potential V_(ss)and the potential V_(dd). Therefore, the potential of the wiring 225 canbe prevented from deviating from this range greatly, and a circuit in asubsequent stage of the protection circuit (a circuit, an input portionof which is electrically connected to V_(out)) can be protected frombeing damaged or deteriorating. Further, when a protection circuit isprovided for an input terminal, potentials of all the wirings suppliedwith a signal can be kept constant (here the potential V_(ss)) when asignal is not input. That is, when a signal is not input, the protectioncircuit also functions as a short-circuit ring capable ofshort-circuiting the wirings. Therefore, electrostatic breakdown causedby a potential difference between the wirings can be prevented. Inaddition, since the resistance value of the resistor 224 is sufficientlylarge, decrease in potential of a signal supplied to the wiring 225 canbe prevented at the time of inputting the signal.

The protection circuit illustrated in FIG. 10C is a protection circuitin which two n-channel thin film transistors are used for each of theprotection diode 220 and the protection diode 221.

Note that although diode-connected n-channel thin film transistors areused for the protection diodes in the protection circuits illustrated inFIGS. 10B and 10C, the present invention is not limited to thisstructure.

The protection circuit illustrated in FIG. 10D includes protectiondiodes 230 to 237 and a resistor 238. The resistor 238 is connected inseries between the wiring 239A and the wiring 239B. A diode-connectedn-channel thin film transistor is used for each of the protection diodes230 to 233. In addition, a diode-connected n-channel thin filmtransistor is used for each of the protection diodes 234 to 237.

The protection diode 230 and the protection diode 231 are connected inseries, one end thereof is kept at the potential V_(ss), and the otherend thereof is connected to the wiring 239A at the potential V_(in). Theprotection diode 232 and the protection diode 233 are connected inseries, one end thereof is kept at the potential V_(dd), and the otherend thereof is connected to the wiring 239A at the potential V_(in). Theprotection diode 234 and the protection diode 235 are connected inseries, one end thereof is kept at the potential V_(ss), and the otherend thereof is connected to the wiring 239B at the potential V_(out).The protection diode 236 and the protection diode 237 are connected inseries, one end thereof is kept at the potential V_(dd), and the otherend thereof is connected to the wiring 239B at the potential V_(out).

The protection circuit illustrated in FIG. 10E includes a resistor 240,a resistor 241, and a protection diode 242. Although a diode-connectedn-channel thin film transistor is used for the protection diode 242 inFIG. 10E, the present invention is not limited to this structure. Aplurality of diode-connected thin film transistors may be used. Theresistor 240, the resistor 241, and the protection diode 242 areconnected to a wiring 243 in series.

The resistor 240 and the resistor 241 can relieve a steep change in thepotential of the wiring 243 and can prevent deterioration or breakdownof a semiconductor element. Further, the protection diode 242 canprevent reverse bias current from flowing through the wiring 243 due tothe change in potential.

Note that the protection circuit illustrated in FIG. 10A can be replacedwith a structure illustrated in FIG. 10F. FIG. 10F illustrates astructure in which the protection diode 211 and the protection diode 212in FIG. 10A are replaced with a protection diode 216, and the protectiondiode 213 and the protection diode 214 are replaced with a protectiondiode 217. In particular, since the diode which is described in theabove embodiment has high withstand voltage, the structure asillustrated in FIG. 10F can be used.

Note that when only the resistors are connected to the wiring in series,a steep change in the potential of the wiring can be relieved, anddeterioration or breakdown of a semiconductor element can be prevented.Further, when only the protection diodes are connected to the wiring inseries, reverse current can be prevented from flowing through the wiringdue to the change in potential.

Note that the protection circuit provided in the display device which isone embodiment of the present invention is not limited to the structuresillustrated in FIGS. 10A to 10F, and design of the protection circuitcan be changed as appropriate as long as the protection circuit has acircuit configuration having a similar function.

Embodiment 8

The display device including the protection circuit described inEmbodiment 7 can be applied to an electronic device.

As examples of the electronic device in which the display device ofEmbodiment 7 is applied to a display portion, the following can begiven: cameras such as video cameras and digital cameras, goggle typedisplays, navigation systems, audio replay devices (e.g., car audiosystems and audio systems), computers, game machines, portableinformation terminals (e.g., mobile computers, mobile phones, portablegame machines, and electronic book readers), image replay devices inwhich a recording medium is provided (specifically, devices that arecapable of replaying recording media such as digital versatile discs(DVDs) and equipped with a display that can display an image), and thelike.

A display illustrated in FIG. 11A includes a housing 300, a support 301,and a display portion 302, and has a function of displaying a variety ofinput information (e.g., still images, moving images, and text images)on the display portion 302. Note that the function included in thedisplay illustrated in FIG. 11A is not limited to this example, and forexample, the display may be provided with a speaker, or the display maybe a touch panel through which information can be not only displayed butinput.

In a television set illustrated in FIG. 11B, a display portion 312 isincorporated in a housing 311. The display portion 312 can displayimages. Here, the structure in which the rear side of the housing issupported by being fixed to a wall 310 is shown.

The television set illustrated in FIG. 11B can be operated with anoperation switch of the housing 311 or a remote controller 315. Channelsand volume can be controlled with an operation key 314 of the remotecontroller 315 so that an image displayed on the display portion 312 canbe controlled. Further, the remote controller 315 may be provided with adisplay portion 313 for displaying data output from the remotecontroller 315.

Note that the television set illustrated in FIG. 11B may be providedwith a receiver, a modem, and the like. With the use of the receiver,general television broadcasting can be received. Moreover, when thetelevision set is connected to a communication network with or withoutwires via the modem, one-way (from a sender to a receiver) or two-way(between a sender and a receiver or between receivers) informationcommunication can be performed.

A computer illustrated in FIG. 11C includes a main body 320, a housing321, a display portion 322, a keyboard 323, an external connection port324, and a pointing device 325, and has a function of displaying avariety of information (e.g., still images, moving images, and textimages) on the display portion 322. Note that the function of thecomputer illustrated in FIG. 11C is not limited to this example, and forexample, may include a function of a touch panel capable of inputtinginformation as well as displaying information.

As described in this embodiment, the diode which is one embodiment ofthe present invention can be applied to the display device.

This application is based on Japanese Patent Application serial no.2009-251186 filed with Japan Patent Office on Oct. 30, 2009, the entirecontents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE

101: substrate, 103: insulating film, 105: first electrode, 106: firstelectrode, 107: oxide semiconductor film, 109: second electrode, 111:gate insulating film, 113: third electrode, 115: third electrode, 117:insulating film, 119: contact hole, 121: contact hole, 123: contacthole, 125: wiring, 129: wiring, 131: wiring, 132: wiring, 133: thin filmtransistor, 141: thin film transistor, 143: thin film transistor, 145:thin film transistor, 151: oxide semiconductor film, 153: dashed lineportion, 155: amorphous region, 157: crystal grain, 200: substrate, 201:pixel portion, 202: input terminal, 203: input terminal, 204: protectioncircuit, 205: protection circuit, 206: protection circuit, 207:protection circuit 208: wiring, 209: wiring, 211: protection diode, 211a: n-channel thin film transistor, 211 b: n-channel thin filmtransistor, 212: protection diode, 213: protection diode, 214:protection diode, 215: wiring, 218: wiring, 220: protection diode, 221:protection diode, 222: capacitor, 223: capacitor, 224: resistor, 225:wiring, 230: protection diode, 231: protection diode, 232: protectiondiode, 233: protection diode, 234: protection diode, 235: protectiondiode, 236: protection diode, 237: protection diode, 238: resistor,239A: wiring, 239B: wiring, 240: resistor, 241: resistor, 242:protection diode, 243: wiring, 300: housing, 301: support, 302: displayportion, 310: wall, 311: housing, 312: display portion, 313: displayportion, 314: operation key, 315: remote controller, 320: main body,321: housing, 322: display portion, 323: keyboard, 324: externalconnection port, and 325: pointing device.

The invention claimed is:
 1. A non-linear element comprising: a firstelectrode over a substrate; an oxide semiconductor film on and incontact with the first electrode; a second electrode on and in contactwith the oxide semiconductor film; a gate insulating film covering thefirst electrode, the oxide semiconductor film, and the second electrode;a plurality of third electrodes adjacent to the oxide semiconductor filmwith the gate insulating film interposed therebetween, the plurality ofthird electrodes being in contact with the gate insulating films andoverlying a plurality of channel regions in the oxide semiconductorfilm, each of the plurality of channel regions having a sameconductivity type; an insulating film over the plurality of thirdelectrodes; an opening in the gate insulating film and the insulatingfilm; and a wiring connected to the second electrode via the opening,wherein the wiring is connected to each one of the plurality of thirdelectrodes, wherein the plurality of third electrodes are connected tothe second electrode, and wherein a part of the plurality of thirdelectrodes is over the second electrode.
 2. The non-linear elementaccording to claim 1, wherein the first electrode functions as one of asource electrode and a drain electrode, wherein the second electrodefunctions as the other of the source electrode and the drain electrode,and wherein the plurality of third electrode functions function as agate electrode.
 3. A non-linear element comprising: a first electrodeover a substrate; an oxide semiconductor film on and in contact with thefirst electrode; a second electrode on and in contact with the oxidesemiconductor film; a gate insulating film covering the first electrode,the oxide semiconductor film, and the second electrode; a gate electrodeadjacent to the oxide semiconductor film with the gate insulating filminterposed therebetween, the gate electrode being in contact with thegate insulating film and overlying a plurality of channel regions in theoxide semiconductor film, each of the plurality of channel regionshaving a same conductivity type; an insulating film over the gateelectrode; an opening in the gate insulating film and the insulatingfilm; and a wiring connected to the second electrode via the opening,wherein the wiring is connected to the gate electrode, wherein the gateelectrode is connected to the second electrode, wherein the firstelectrode functions as one of a source electrode and a drain electrode,wherein the second electrode functions as the other of the sourceelectrode and the drain electrode, and wherein a part of the gateelectrode is over the second electrode.
 4. The non-linear elementaccording to any one of claims 1 and 3, wherein the oxide semiconductorfilm comprises hydrogen at a concentration of 5×10¹⁹ atoms/cm³ or less.5. The non-linear element according to any one of claims 1 and 3,wherein the oxide semiconductor film has a carrier concentration of5×10¹⁴ atoms/cm³ or less.
 6. The non-linear element according to any oneof claims 1 and 3, wherein at least a portion of the gate insulatingfilm is in contact with the oxide semiconductor film, and wherein theportion of the gate insulating film is an oxide insulating film.
 7. Thenon-linear element according to claim 6, wherein the oxide insulatingfilm is a silicon oxide film, and wherein the silicon oxide film iscovered with silicon nitride.